More than 100 people have been killed and 1,500 injured at a protest held by supporters of ousted Egyptian President Mohammed Morsi in Cairo, doctors say.
However the health ministry has put the death toll lower, at 38.
The army ousted Mr Morsi on 3 July. He has been formally accused of murder, relating to a 2011 jail outbreak, and of links to the militant group Hamas.
Both pro- and anti-Morsi demonstrators held huge protests overnight in the capital.
The anti-Morsi camp occupied Cairo's Tahrir Square in support of the army, after its chief, Gen Abdel Fattah al-Sisi, had urged people to demonstrate to provide a mandate for its intervention.
Meanwhile, tens of thousands of Morsi supporters continued their sit-in protest at the mosque in the Nasr City area.
On Saturday, Interior Minister Mohammed Ibrahim vowed to end the sit-in, saying local residents had complained about the encampment.
He said the protest would be "brought to an end soon, and in a legal manner" with an order from the prosecutor, although this has yet to happen.
The BBC's Jim Muir in Cairo says the latest violence is the most serious since the army's intervention to remove President Morsi, but this does not appear to have been a planned campaign to clear the area around the mosque.
'Shooting to kill'
It appears that clashes began after some of the Morsi supporters tried to block a main road in the area, and security forces responded.
The state news agency Mena quotes a security official as saying they had been trying to stop fighting between rival sides, and that eight security personnel had been injured.
The official added that live fire had not been used, only tear gas.
But our correspondent says medics at the hospital believed about 70% of the casualties were caused by live fire - with many of the victims hit in the chest or head by snipers firing from rooftops.
Ahmed Nashar, a Brotherhood spokesman witnessed what happened near the Nasr City mosque where demonstrators built a wall to protect themselves.
"When I arrived, bullets were whizzing past my ears," he told the BBC. "Today was just brutal - people were fired at, with live firearms."
Our correspondent says Morsi supporters are furious about the role the military is taking, and in particular the head of the army, General Abdel Fattah al-Sisi, whom they say is killing Egyptians.
Egypt's pro-Muslim Brotherhood TV station, Ahrar 25, quoted the coalition that supports Mr Morsi - the National Coalition for Supporting Legitimacy - as saying it held Gen Sisi responsible for the deaths at the mosque protest.
There has also been violence in Egypt's second city of Alexandria, where at least 10 people have been killed in clashes between rival factions.
European Union foreign policy chief Catherine Ashton said she "deeply deplored" the latest deaths in Egypt.
Her spokesman called "on all actors to refrain from violence and to respect the principles of peaceful protest".
UK Foreign Secretary William Hague said: "Now is the time for dialogue, not confrontation. It is the responsibility of leaders on all sides to take steps to reduce tensions."
Mr Morsi, Egypt's first democratically elected president, has been formally remanded in custody at an undisclosed location for an initial 15-day period, according to a judicial order on Friday.
The order was the first official statement on Mr Morsi's legal status since he was overthrown.
He has now been formally accused of the "premeditated murder of some prisoners, officers and soldiers" when he and several Muslim Brotherhood leaders were freed during a breakout at a Cairo prison in January 2011.
He is alleged to have plotted attacks on jails in the uprising that overthrew President Hosni Mubarak.
Mr Morsi is also accused of conspiring with the Palestinian militant group Hamas, which governs the Gaza Strip and has strong links with the Muslim Brotherhood.
Two human rights activists visited Mr Morsi's place of detention on Friday, although he reportedly refused to meet them, instead sending his chief of presidential staff Refaa el-Tahtawy - who is also held at the location.
Mr Tahtawy told the visitors they were being treated well but complained that they were unable to meet relatives.
On Saturday, the interior minister Ibrahim said that Mr Morsi would be transferred to Torah Prison, where Mubarak is being held.